Precious metal catalysts are used extensively in the petrochemical industry. Because of their functionality, precious metal catalysts are also likely to play a significant role in future bio-refineries. The main drawback of these catalysts, of course, is their very high price. For example, the spot price for gold crested $1000/ounce in September 2009, passed $1500/ounce in March of 2010, and as of February 2013 is trading at approximately $1600/ounce. Silver and platinum prices have experienced similar increases and as of February 2013 are trading near their all-time highs. While it would be desirable to replace these precious metal catalysts with more abundant base metals such as copper, nickel, or iron, these base metal catalysts are subject to deactivation by leaching and sintering under condensed-phase reaction conditions. (1-4)
Atomic layer deposition (ALD) is a self-limiting, sequential surface chemistry that deposits a conformal, thin-film of material onto a substrate, even on substrates having high aspect ratios. (“Conformal” in the context of ALD means that the thin film deposited by ALD has a substantially uniform thickness everywhere along the coated substrate.) ALD enables the deposition of atomic-scale thin-films of controlled thickness. ALD is similar in chemistry to chemical vapor deposition (CVD), but separates the deposition reaction into two half-reactions that are performed separately from one another. In this fashion, the thickness of the deposited film can be very accurately controlled. See, for example, Steven M. George (2010). “Atomic Layer Deposition: An Overview”. Chem. Rev. 110 (1): 111-131.
Chemical vapor deposition (CVD) refers to a series of closely-related processes in which a substrate is exposed to one or more volatile precursors which react with and/or decompose on the substrate surface to produce a conformal thin film. As noted, CVD is practiced in a wide variety of formats, all of which share the same basic feature of bringing the volatile precursors into contact with a substrate to deposit an atomically thin layer of a desired material. The various types of CVD can be classified in various ways, such as by the operating pressure of the process or by the means by which the chemical reactions are initiated to form the thin film on the substrate. Thus, there are known in the art atmospheric pressure CVD (APCVD), low-pressure CVD (LPCVD) (CVD at sub-atmospheric pressures; reduced pressures tend to reduce unwanted gas-phase reactions and improve film uniformity), and uiltrahigh vacuum CVD (UHVCVD) (CVD at very low pressure, typically below 10−6 Pa). CVD may also be classified by the physical characteristics of reactive vapor. Thus, known to the art are CVD processes including aerosol-assisted CVD (AACVD). Here, the precursors are transported to the substrate by means of a liquid/gas aerosol. AACVD is suitable for use with non-volatile precursors. In direct liquid injection CVD (DLICVD) the precursors are in liquid form (liquid or solid dissolved in a convenient solvent) and injected in a vaporization chamber and then transported to the substrate. High growth rates can be reached using DLICVD. CVD methods may also be characterized by how the plasma vapor is formed or maintained. Thus, known to the art are various types of CVD such as microwave plasma-assisted CVD (MPCVD) and plasma-enhanced CVD (PECVD). PECVD utilizes plasma to enhance chemical reaction rates of the precursors and also allows deposition of the thin film at lower temperatures, which could be critical for temperature-sensitive materials. Remote plasma-enhanced CVD (RPECVD) is similar to PECVD except that the substrate is not directly in the plasma discharge region. Removing the substrate from the plasma region allows processing temperatures to drop even further, even down to room temperature with certain thin films. Atomic layer CVD (ALCVD) enables depositing successive layers of different substances to produce layered, crystalline films. ALCVD is also known as atomic layer epitaxy.
Combustion Chemical Vapor Deposition (CCVD) is an open-atmosphere, flame-based technique for depositing high-quality thin films and nanomaterials. Hot wire CVD (HWCVD), also known as catalytic CVD (Cat-CVD) or hot filament CVD (HFCVD), uses a hot filament to chemically decompose the source gases which are then contacted with the substrate to be coated. Hybrid Physical-Chemical Vapor Deposition (HPCVD)—involves both chemical decomposition of a precursor gas and vaporization of a solid source to yield a reactive vapor that then forms the coating on the substrate. Metalo-organic chemical vapor deposition (MOCVD) is based on metalo-organic precursors. Rapid thermal CVD (RTCVD) uses heating lamps or other means to heat the substrate very rapidly. Heating only the substrate rather than the reactive gas or chamber walls helps reduce unwanted gas-phase reactions that can lead to particle formation.
Coking- and sintering-resistant palladium catalysts have been described for use in gas-phase heterogeneous reactions. (14) Here, the authors noted that overcoating of supported metal nanoparticles effectively reduced deactivation and coking in high-temperature, gas-phase applications of heterogeneous catalysts. In this paper, a palladium catalyst was overcoated with 45 layers of alumina via ALD. The coated catalysts were then used for 1 hour in oxidative dehydrogenation of ethane to ethylene at 650° C. Coking of the coated palladium catalyst was greatly reduced. Scanning transmission electron microscopy revealed that the morphology of the coated catalyst was not changed after the ethane dehydrogenation reaction was run at 675° C. for 28 hours. Coating the palladium catalyst with alumna improved the yield of ethylene as compared to the non-coated catalyst. The reactions described in this work are gas-phase only, and used only palladium catalysts (a noble metal catalyst). Using a base metal catalyst in condensed-phase conditions is significantly different from using noble metals in either the gas phase or condensed phase due to the possibility of leaching.